45 / 2023-04-30 22:21:10

The influence and process of mineral coating on carbon capture of biochar via aerobic carbonization with water-fire coupled method

Carbon capture，Water-fire coupled methods, lime-water coated cladding, Exposure times

As a carbon-rich material with porous structure, biochar has multiple functions including carbon sequestration, soil quality improvement and fertility enhancement, as well as immobilization of heavy metals and organic pollutants. However, large-scale use of biochar in agricultural production and environmental remediation, has been constrained by its low production and high cost until now. To assume, if biochar could be produced in situ from local bio-waste for local use, its transportation cost would be eliminated, and overall cost would be greatly decreased, thus making biochar use as a carbon-negative scheme more feasible. To this end, the hard part of this project is how to create a oxygen-limited condition in the field. In this study, a new approach of limewater-coated cladding was used as mineral coating to create oxygen-limited conditions; and then, Zea mays L. cob was used as a feedstock to produce low-cost biochar in the field via a combination of aerobic and mineral coating oxygen-limited carbonization with a fire-water coupled method; selectively, we priority investigated the exposure times and/or mineral coating on the influence of carbon capture of biochar. The characteristics of biochar thus produced were deeply affected by exposure times (the gap between a burning charcoal fell to the ground and being extinguished by water spray). Specifically, with the increased of exposure time, the carbon content, specific surface area, as well as function groups (i.e. phenolic-OH and -COOH) were all decreased, respectively; and biochar formed by zero exposure time showed a larger carbon conversion rate (18.92%); further, carbon conversion rate of Zea mays L. cob enhanced to 35.87% under the mineral coating. The mechanisms of this process may involving the followings aspects: 1) the mineral coating of limewater-coated cladding created a oxygen-limited condition, which increased the capacity of carbon capture of biochar; 2) and the Ca²⁺ combined with function groups (i.e. phenolic-OH and -COOH) via ration bridging, which improved the carbon content; 3) moreover, benefited from complexation of Ca²⁺ derived from limewater and DOM (dissolved organic matter) of biochar, which ulteriorly enhance carbon capture; 4) meanwhile, π-electron interaction: reacted with aromatic carbon also strengthened carbon sequestration. To help understand this process, we can conceive a Zea mays L. cob as a miniature furnace: the outer part of the mineral coating of limewater-coated cladding in the cob is similar to the furnace wall, whereas the cob is equivalent to the biomass in the furnace. In other words, the carbonization process is a combination of mineral coating via limewater-coated cladding created a oxygen-limited pyrolysis at the cob and water-spray formed the biochar; during the process, Ca²⁺ prevented the fracture of C=O and O=C-O bonds to produce COx in variety of ways for improving the carbon capture and optimizing the carbon structure of biochar which derived from Zea mays L. cob. This study may help transfer a paradigm shift in biochar production from a sophisticated stationary facility to a simplified method for practical use in situ. Outcomes from this research will underpin the use of biochar as a cost-effective carbon-negative technology to help achieve carbon neutrality.